This invention relates to an electrochemical cell and to methods of fabricating the cell and improving its capacity and/or power and capability of operating at low temperatures. The invention also relates to a new positive electrode or cathode during discharge for electrochemical cells and method of fabricating same, and more particularly, relates to electrochemical cells and positive electrodes for metal chloride batteries having lower internal impedance and greater discharge capacity with a higher specific energy and power.
According to the invention, an electrochemical cell comprises an alkali metal, and preferably, a sodium negative electrode or anode during discharge which is molten at operating temperatures of the cell, an alkali and preferably, a Na.sup.+ ion conducting solid electrolyte/separator, a molten salt liquid electrolyte in the positive electrode compartment which is compatible with the positive electrode, and which is also at least partially molten at the operating temperature of the cell, and a positive electrode which is impregnated by the liquid electrolyte and which comprises, as the electrochemically active positive electrode substance of the cell, a transition metal chloride which preferably is selected from the group consisting or iron chloride, nickel chloride, chromium chloride, cobalt chloride and manganese chloride or mixtures thereof. Since the cell with a Na electrode has received the major development effort, a shorthand method of referring to these cells is (Na/MCl.sub.2) battery or electrochemical cell, wherein M is one of the transition metals identified above. Batteries of this type are disclosed in U.S. Pat. No. 4,288,506 issued Sep. 8, 1981, to Coetzer et al. and U.S. Pat. No. 4,546,055 issued Oct. 8, 1985 to Coetzer et al. and U.S. Pat. No. 4,592,969 issued Jun. 3, 1986 to Coetzer et al. The batteries or electrochemical devices of the type herein discussed are useful as a power source alternative to petroleum engines and are being developed commercially, not only for electrically powered vehicles, but also for load leveling in electrical utilities.
An ideal electrochemical cell or battery should exhibit a number of characteristics, including low resistance and high discharge rates, operation over a wide temperature range, a capability to operate over a large number of cycles, and high energy on a volume, weight and cell basis. Generally, these types of electrochemical cells or batteries consist of two dissimilar metals in an ionically conductive medium, with the ionization potential of one metal sufficiently higher than the other metal to yield a voltage upon reduction/oxidation redox (coupling) over and above that needed to break down the electrolyte continuously at the positive electrode.
Metal typically goes into solution at the negative electrode or anode, releasing electrons to travel in the external circuit to the positive electrode, or cathode, doing work in transit. Material which will go through a valency drop on electrochemical discharge is included in the positive electrode. In essence, this material, the oxidizer, accepts electrons coming from the negative electrode and serves as the depolarizer. The depolarizer or cathode is positioned, in one embodiment, in the positive electrode in combination with some electrolyte-containing matrix, and should be porous to allow access of the electrolyte to the enlarged area of the depolarizer or cathode. Porosity of the cathode provides a surface at which the redox reaction may take place.
The economic and social advantages of powering automobiles from batteries are considerable as the vehicles could operate at relatively high efficiencies, such as 30-40%, and be non-polluting. Two important characteristics are considered in seeking an energy storage system for a vehicle. One of the characteristics or variables, specific power, designated in watt per kilogram (W/kg), determines to a large extent, acceleration and speed capabilities. The other consideration or variable of specific energy is designated as watt hours per kilogram (Wh/kg), determines vehicle range. The capacity density of a cell, or how much electrochemical energy the electrode will contain per until volume is designated as ampere hours per cubic centimeter (Ah/cm.sup.3).
It is generally seen, therefore, that increasing the cell capacity available during discharge and the cell power by lowering the internal impedance of the cell are both important attributes in the consideration of how and when and to what extent electrochemical cells will be placed in the vehicle as a significant portion of the vehicle propulsion systems.
Sodium/metal chloride cells of the type disclosed in the patents hereinbefore identified use a sodium anode, a .beta." alumina solid electrolyte and a cathode designated as MCl.sub.2 with a molten electrolyte of sodium chloroaluminate, NaAlCl.sub.4.
Metal halide batteries exploit the higher electrolysis threshold values of the electrolyte constituents. In charging, the positive electrode becomes poor in sodium salt with sodium metal being deposited on the negative electrode and the halogen electrochemically reacting with the metal to form a metal halide. Among halides, the fluorides and chlorides exhibit higher electrolysis thresholds than bromide and iodides, and therefore are preferred and generally used. As such, metal chloride and metal fluoride systems exhibit relatively higher energy densities and lighter mass than systems using bromides and iodides. Because of the better electrochemical properties and low price, the metal chloride systems are preferred.
As with other electrochemical cells, metal halide batteries generate electricity by transporting electrons from the fuel constituent to the oxidizer, with concomitant oxidation and reduction occurring at the negative electrode or anode and the positive electrode or cathode, respectively. The following reaction occurs: EQU MX.sub.2 +2 Na.rarw..fwdarw.2 NaX+M
where M is a transition metal and preferably is one or more of nickel, iron, cobalt, chromium and manganese and X is a halogen, preferably chlorine. The left hand side of the above equation depicts a charged state, before reduction of the metal halide, with the right hand side of the equation depicting a discharged state with reduced transition metal.
Utilization of the metal/chloride system is usually expressed on the basis of the ratio of the reacted NaCl to the total quantity of NaCl used to fabricate the positive electrode. This practice is convenient for the Na/MCl.sub.2 cell because they are fabricated in the discharge state and the MCl.sub.2 active material is formed electrochemically, as noted in the above cell reaction. As used hereinafter, weight percent of a constituent in the positive electrode refers to the positive electrode in the dry state, as the electrodes exist prior to being placed in the electrochemical cell and cycled to charge the cell.
One of the significant problems in the sodium metal halide batteries is the limited battery capacity, due to the chloride of the positive electrode metal which forms a layer of low conductivity on the positive electrode. Since this metal chloride has limited conductivity, after it reaches a certain thickness on the order of one micrometer, it practically terminates further charge uptake of the cell. It has also been noted that cell capacity may be lowered after repeated charge and discharge cycles. Previous efforts to improve cell performance have involved the addition of sulfur to the liquid electrolyte or the addition of sulfide to the porous positive electrode. Neither of these solutions has been totally satisfactory.